The present invention is primarily focused on taking accurate measurements of volumetric water content (VWC) in a variety of media, including but not limited to: soil, wood, concrete, building materials, clothing, and the like. This invention is useful in such applications as crop irrigation, lawn care, gardening and other landscaping applications, laundry systems, non-destructive testing for ground water infiltration into buildings, and a full range of moisture environments ranging from dry to completely saturated.
The present invention is particularly suited for big data environments such as large scale deployment of numerous sensors such as industrial irrigation systems and municipal irrigation systems because the present invention addresses the issues of accuracy and total lifecycle cost of soil monitoring devices. The incident methods are intended for but not limited to synthesis into microchips.
In relation to capturing a set of soil vital signs in a big data environment there is particular interest on the accurate measurement of volumetric water content in soil at large numbers of zones. However, this invention has wider applicability.
The present invention sets forth a low-cost, ultra-low power moisture sensor and integrated communication system to make remote wireless applications possible which utilize continuous unattended monitoring of a multiplicity of locations or zones. Further, the present invention provides for the implementation of inexpensive sensors with improved longevity, durability, security, reliability and wide spread applicability.
Measuring soil moisture content at a municipal or industrial scale is complex and includes a number of well-known problems in order to measure to an adequate level of accuracy and reproducibility at a reasonable cost. Often at the municipal and industrial level, deployment of a large number of sensors impacts the overall system cost because of the requirement to frequently replace batteries and the need to dean sensor probes to ensure adequate accuracy. In addition, the large number of sensors must work in a wide range of soil types and soil conditions from dry to fully saturated.
There are a number of known techniques for measuring soil water content including (a) Neutron probe—uses radioactive material which is expensive and is typically inaccurate in topsoil (b) Matric potential—uses low cost gypsum which has slow response and lacks durability (c) Tensiometers—require regular maintenance (d) Time Domain Reflectometer—is expensive and is only accurate up to 65% VWC (e) Capacitive—is low cost but is susceptible to electrical conductivity issues (f) Frequency—low cost but is susceptible to electrical conductivity issues and is limited in range (g) Impedance Matching—expensive and limited range.
The following references are representative of some of the known devices and techniques for measuring soil water content.
U.S. Pat. No. 7,944,220, teaches the performance of a dielectric moisture content sensor which is commonly limited by sensitivity to salinity and nutrient levels in the soil, as well as sensitivity to temperature change. This is in part due to soil non-homogeneity and variation of soil composition. Most crops are grown in soil with a salinity and nutrient level corresponding to an electrical conductivity between 60 mS/m to 400 mS/m and as high as 500 mS/m to 600 mS/m for certain crops, such as tomatoes, with soil conductivity in coastal environments up to 3,000 mS/m. Temperature also creates large variations and must be considered in any viable method. Salinity and other nutrients are the primary cause of significant deviation and must be considered. The result of these issues it that most reasonably priced solutions do not function or measure VWC above 65% water saturation.
U.S. Pat. No. 5,424,649, is representative of a common technique used in low cost sensors that have a thin dielectric coating. The dielectric coating is only partially effective at reducing sensitivity to soil conductivity which results in a moisture content sensor with sensitivity to soil conductivity and salinity. Coatings are also subject to wear and do not address the issue of charge stealing by the earth.
U.S. Pat. No. 5,859,536 is representative of common techniques in more costly sensors which use impedance matching networks. Because these techniques depend on current flow into the earth, it is naturally susceptible to conductance of soil to ambient ground.
U.S. Pat. No. 5,804,976 is a more reliable technique utilizing a transmission line and measuring propagation delay. However, this technique also suffers near total loss of signal at high saturation.
U.S. Pat. No. 7,030,630 is a moisture sensor with a capacitive moisture measuring element and method of determining air humidity describes time constants associated with a parallel resistance but is silent regarding the parasitic series resistance.
Current models and consequently the methods currently in use are effective for measuring humidity as in U.S. Pat. No. 7,030,630 but do not function in situ of greater that 65% volumetric water content material.
U.S. Pat. No. 5,730,165 teaches that if the sensor employs an RC circuit or variation thereof, the stray conduction path will rob the plate of charging current and will thus alter its apparent time constant.
Studies including the IEEE Experimental Electrical Modeling of Soil for In Situ Soil Moisture Measurement 2013 are instructive and provide a more accurate electrical model for soil over the entire range of saturation. This model, however, must be adapted to accurately represent the electrical parameters of sensors embedded in the earth.
Often the previously known sensors require battery replacement each season which is a significant limitation to large scale deployment.
Cost of the sensors and vulnerability of current sensors to tampering further prohibits wide deployment in unsecured areas.
As a result, all of the current techniques have proven uses in particular segments but they suffer at least in terms of cost, complexity or accuracy, thus preventing ubiquitous adoption for large scale data collection such as in a metropolitan or industrial sensor network.